feat: add page offset functions in double page layout

[dyphire]

Sometimes the splicing and combination of pages in a double-page layout cannot be achieved simply by switching the presence or absence of a cover. It is necessary to further add the page offset function to achieve a more flexible combination.
It can be added to the long press action, like Aidoku does: Aidoku/Aidoku#697

[everpcpc]

auto mode already did this.

[dyphire]

auto mode already did this.

I don't find this fuction.

The expectation should be:

[everpcpc]

Oh, I mean auto mode auto detect page dimension, and display horizontal image as a single page.

[dyphire]

Oh, I mean auto mode auto detect page dimension, and display horizontal image as a single page.

I know that feature, this request is for another page offset feature.

[everpcpc]

OK I got it.